Hi,
Thanks for the post.
In this case, I suggest we refer to the following articles to troubleshoot this issue:
Deployment Recommendations for Connection Limits in ISA Server 2004
http://technet.microsoft.com/en-us/library/cc302445.aspx
Deployment Guidelines for ISA Server 2004 Enterprise Edition
http://technet.microsoft.com/en-us/library/cc302471.aspx
As it is a ISA-related issue, please get further assistance from the following link:
Forefront Edge Security Community
http://technet.microsoft.com/en-us/forefront/edgesecurity/bb687298.aspx
Hope this helps.